The invention relates to ethylene-vinyl acetate copolymers, to a process for preparing them, to their use as flow improvers, especially in petroleum middle distillates, such as diesel fuels and light heating oils, and to fuel compositions comprising them.
Ethylene-vinyl acetate copolymers have been employed for some time as flow improvers in petroleum middle distillates. Their function in such distillates is, in particular, to lower the cold filter plugging point (CFPP), which is determined in accordance with EN 116. Preparation of the ethylene-vinyl acetate copolymers is generally by polymerization of the monomers using the high-pressure technique. US 3,627,838 describes a process for preparing pour-point improvers which uses ethylene-vinyl acetate copolymers. Ethylene and vinyl acetate are reacted at from 138 to 171xc2x0 C. and under from 48 to 137 bar in a reactor.
DE-A 25 15 805 describes ethylene copolymers, processes for preparing them, and distillate oils comprising them. The reaction of ethylene and vinyl acetate takes place in an autoclave in cyclohexane as solvent. The temperature during the reaction is about 105xc2x0 C., and the pressure about 75 kp/cm2.
EP-A-0 007 590 describes petroleum middle distillates of enhanced filterability. The petroleum distillates contain, for example, ethylene-vinyl acetate copolymers which are prepared solventlessly from the monomers at from 100 to 350xc2x0 C. and under from 500 to 2000 bar. The polymers are employed together with a monomer such as vinyl acetate as flow improvers.
The use of stirred autoclaves, in particular, is established in the industrial preparation of the copolymers. The use of tube reactors without backmixing leads in general to more heterogeneous products, since along the tube reactor the reaction mixture passes several times through areas of relatively large increase in temperature following the additions of initiator. The resulting copolymers generally have a broad molecular weight distribution.
It is an object of the present invention to provide a process for preparing ethylene and vinyl acetate copolymers which leads to copolymers having a narrow molecular weight distribution that can be employed with advantage as flow improvers in petroleum middle distillates.
We have found that this object is achieved by providing a process for preparing ethylene-vinyl acetate copolymers in which a mixture of ethylene and vinyl acetate is polymerized continuously in the presence of free-radical polymerization initiators and, if desired, molecular weight regulators in a cooled tube reactor at a pressure in the range from 1000 to 3500 bar, preferably 1200 bar, and where the polymerization initiator is added at the beginning and at two or more points along the tube reactor, and temperature changes in the reaction mixture along the tube reactor, between the first temperature maximum and the last addition of the polymerization initiator, are within a range of not more than 20xc2x0 C.
It has been found that ethylene-vinyl acetate copolymers having advantageous properties, especially narrow molecular weight distributions, are obtainable if the temperature of the reaction mixture in the tube reactor is held within a very narrow range.
The reaction regime should therefore be as homogeneous as possible along the tube reactor. Normally, a polymerization initiator and, if desired, a molecular weight regulator are added to a monomer mixture of ethylene and vinyl acetate upstream of the tube reactor, and the resulting mixture is polymerized in the tube reactor. At the beginning of the reaction, the temperature first of all increases, before falling again owing to the cooling of the tube reactor. In general, further polymerization initiator is supplied at two or more points along the tube reactor in order to obtain maximum conversions within the tube reactor. Following each addition of the polymerization initiator there is first of all a further increase in temperature, which then falls owing to cooling. It has been found that a rapid sequence of additions of initiator with subsequent short cooling zones along the tube reactor allow a homogeneous reaction regime, i.e. small temperature differences, thereby giving a copolymer having a narrow molecular weight distribution.
With a given level of cooling, the respective amount of the polymerization initiator and the sites of the addition of the initiator along the tube reactor are preferably chosen so that between the first temperature maximum and the last addition of the polymerization initiator the average maximum temperature in the reaction mixture is not more than 15xc2x0 C., especially not more than 10xc2x0 C., above the average minimum temperature.
Prior to the first heating, the reaction mixture has a low temperature, which is present when the monomers are mixed. Following the last addition of the polymerization initiator, the reaction mixture is cooled in order to enable the product to be discharged from the reactor. Consequently, in accordance with the invention, the temperature level in the reaction mixture is left as constant as possible between the first heating, i.e. the first attainment of the maximum temperature, and the final cooling of the reaction mixture.
The average maximum temperature of the reaction mixture is preferably in the range from 230 to 250xc2x0 C., with particular preference from 235 to 245xc2x0 C. In particular, the average maximum temperature is about 240xc2x0 C. The average minimum temperature is preferably about 230xc2x0 C.
Following the initiation at the beginning of the reactor, the internal temperature in the tube reactor rises with particular preference to about 240+/xe2x88x925xc2x0 C. The reaction mixture then cools owing to the cooling of the tube reactor (preferably by means of water cooling) to about 230xc2x0 C. +/xe2x88x925xc2x0 C. This is followed by further initiaton to an extent such that the temperature of the reaction mixture rises to about 240xc2x0 C. +/xe2x88x925xc2x0 C. Depending on the reactor length, this procedure is repeated a number of times in order to obtain a high conversion.
Depending on the desired conversion it is possible to vary the number of sites along the tube reactor at which polymerization initiator is added. Polymerization initiator is preferably added at 2 to 6, with particular preference at 3 to 5, sites along the tube reactor.
The pressure in the tube reactor is from 1000 to 3500 bar, preferably from 1200 to 2500 bar.
The length/diameter ratio of the tube reactor is preferably in the range from 10,000 to 50,000, with particular preference from 15,000 to 30,000.
The proportion of vinyl acetate in the reaction mixture is preferably from 15 to 45% by weight, with particular preference from 20 to 35% by weight. The remainder of the reaction mixture constitutes ethylene. The copolymer is therefore preferably composedxe2x80x94apart from residues of a free-radical initiator or molecular weight regulatorxe2x80x94of ethylene and vinyl acetate.
Free-radical polymerization initiators which can be employed are any desired such initiators. It is possible to employ peroxides which dissociate to free radicals, preferably organic peroxides, air or oxygen.
Examples of suitable free-radical initiators are organic peroxides, such as peresters, perketals and percarbonates. In particular use is made of tert-butyl perpivalates and/or tert-butyl perisononanoate. The latter can be employed in a molar ratio of from 10:1 to 1:10, preferably from 5:1 to 1:5 and, in particular, about 1:1.
The amount of the free-radical polymerization initiator employed is preferably from 10 to 1000 mol-ppm, with particular preferance from 20 to 100 mol-ppm.
Molecular weight regulators which can be employed are any desired and appropriate molecular weight regulators, such as aliphatic and olefinic hydrocarbons, aliphatic aldehydes, and so on. Particular preference is given to the use of aliphatic aldehydes, especially propionaldehyde. The molecular weight regulator is preferably added to the reaction mixture upstream of the tube reactor. It can also be added together with the polymerization initiator at the various sites along the tube reactor.
The ethylene-vinyl acetate copolymers obtained in accordance with the invention preferably have a quotient Q of weight-average molecular weight (MW) to number-average molecular weight (Mn) of from 2.0 to 2.5. Copolymers prepared by known processes generally have a Q of more than 2.6. The melt viscosity of the copolymers of the invention is preferably from 40 to 95 cst, with particular preference from 60 to 80 cst. The melt viscosity is determined using a rotary viscometer in accordance with DIN 53019 at 120xc2x0 C. The Q values are determined by means of gel permeation chromatography.
With the known copolymers the melt viscosity is more than 100 cst at 120xc2x0 C. Products known to date and having lower melt viscosities are poorer in their solubility or else in-their effectiveness.
The reaction time in the process of the invention is preferably from 60 to 240 s, with particular preference from 60 to 90 s.
The copolymers of the invention are highly effective flow improvers for petroleum distillates, especially petroleum middle distillates, and preferably fuel compositions. They bring about, in particular, a marked reduction in the cold filter plugging point (CFPP) in accordance with EN 116. Moreover, the cloud points, or temperatures at which clouding of an additive-treated fuel begins, are at markedly lower temperatures than in the case of fuels without additives.